The reason for the low serum thyroxine-binding globulin (TBG) in nonthyroid illness (NTI) when measured by an immunoradiometric assay kit was investigated. We found that unsaturated free fatty acids, several non-steroidal anti-inflammatory drugs and the diuretic, furosemide, inhibit thyroid hormone binding. Although the kit can be used to test for binding inhibition, it should not be used to measure the circulating TBG concentration. A simplified low iodine diet was developed for outpatient use prior to I-131 scanning and therapy in thyroid cancer. Iodine intake of five subjects on the diet was approximately 50 mug a day and this level was maintained for four weeks. The diet required only minimal instruction to be followed reliably. This level of iodine intake may increase radioiodine uptake in thyroid carcinomas.